The present invention relates generally to a phase-change memory device, and more particularly, to a multi-bit phase-change memory device.
In general, phase-change materials for use in a memory device refer to materials having a phase that changes according to temperature. The resistance of the phase-change material can vary depending on the state (i.e., the phase) of the phase-change material. For example, in a phase change material the phase of the material can be allowed to change between a crystalline state and an amorphous state with the crystalline state having a resistance that is different from that of the amorphous state. Therefore, the phase-change material can be used as a storage medium in a memory device, since the difference in resistance can be used to define the “states” of information (e.g. the different resistance may be used to define a logic ‘0’ or a logic ‘1’). A memory device including the phase-change material is typically referred to as a phase-change memory device.
A typical phase-change memory device includes a plurality of word lines and a plurality of bit lines that cross (i.e., intersect) the word lines, and typically the point at which a world line and a bit line intersect defines a unit memory cell. A phase-change memory device also typically includes a switching device for selecting the word lines, and a phase-change device cooperating with the switching device in order to store data.
The switching device typically includes a diode, and the phase-change device includes a bottom electrode, a phase-change material layer, and a top electrode. The bottom electrode of the phase-change device is electrically connected to the diode.
Theoretically, the phase-change memory device can obtain logic “0”, logic “1” and an intermediate value according to the phase (for example, the crystalline state) of the phase-change material layer.
However, in order to realize a multi-bit phase change memory device (i.e., a phase change device that can obtain more bits (intermediate values) than simply the logic “0”, logic “1”), the phase-change memory device must be driven in a specific driving scheme different from the driving scheme of the conventional phase-change memory device.
In other words, the phase-change material used for the multi-bit phase change memory device must exhibit a linear characteristic relative to electrical or terminal energy when the state of the phase-change material is changed from the amorphous state to the crystalline state or vice versa. If a phase-change material that does not exhibit a linear characteristic is employed in the memory device, the operational conditions must be determined in the process of reading or writing information of the intermediate state, thereby requiring very complex peripheral circuits for driving the memory device.